Controlling removal rate uniformity of an electropolishing process in integrated circuit fabrication
A metal layer formed on a wafer, the wafer having a center portion and an edge portion, is electropolished by aligning a nozzle and the wafer to position the nozzle adjacent to the center portion of the wafer. The wafer is rotated. As the wafer is rotated, a stream of electrolyte is applied from the nozzle onto a portion of the metal layer adjacent to the center portion of the wafer to begin to electropolish the portion of the metal layer with a triangular polishing profile to initially expose an underlying layer underneath the metal layer at a point.
Latest ACM RESEARCH INC. Patents:
The present application claims the benefit of U.S. Provisional Application No. 60/546,848, filed Feb. 23, 2004, which is incorporated herein by reference in its entirety, and U.S. Provisional Application No. 551,632, filed Mar. 7, 2004, which is incorporated herein by reference in its entirety.
BACKGROUND1. Field
The present application generally relates to an electropolishing process used in integrated circuit (IC) fabrication, and, in particular, to controlling removal rate uniformity during an electropolishing process of a metal layer formed on a wafer used in IC fabrication.
2. Related Art
IC devices are manufactured or fabricated on wafers using a number of different processing steps to create transistor and interconnection elements. To electrically connect transistor terminals associated with the wafer, conductive (e.g., metal) trenches, vias, and the like are formed in dielectric materials as part of IC devices. The trenches and vias couple electrical signals and power between transistors, internal circuits of the IC devices, and circuits external to the IC devices.
In forming the interconnection elements, the wafer may undergo, for example, masking, etching, and deposition processes to form the desired electronic circuitry of the IC devices. In particular, multiple masking and etching steps can be performed to form a pattern of recessed areas in a dielectric layer on a wafer that serve as trenches and vias for the interconnections. A deposition process may then be performed to deposit a metal layer over the wafer to deposit metal both in the trenches and vias and also on the non-recessed areas of the wafer. To isolate the interconnections, such as patterned trenches and vias, the metal deposited on the non-recessed areas of the wafer is removed.
The metal layer deposited on the non-recessed areas of the dielectric layer can be removed using an electropolishing process. In particular, a nozzle can be used to apply an electrolyte solution to electropolish the metal layer. As the feature size of the IC devices continues to decrease, however, the removal rate uniformity of the electropolishing process needs to be enhanced.
SUMMARYIn one exemplary embodiment, a metal layer formed on a wafer, the wafer having a center portion and an edge portion, is electropolished by aligning a nozzle and the wafer to position the nozzle adjacent to the center portion of the wafer. The wafer is rotated. As the wafer is rotated, a stream of electrolyte is applied from the nozzle onto a portion of the metal layer adjacent to the center portion of the wafer to begin to electropolish the portion of the metal layer with a triangular polishing profile to initially expose an underlying layer underneath the metal layer at a point.
BRIEF DESCRIPTION OF THE DRAWINGS
With reference to
In one exemplary embodiment, the electropolishing tool includes a nozzle 106 configured to apply a stream of electrolyte 108 to metal layer 102 at different radial locations on wafer 100. A power supply 110 is connected to nozzle 106 to apply a negative electropolishing charge to stream of electrolyte 108. Power supply 110 is also connected to wafer 100 to apply a positive electropolishing charge to wafer 100. Thus, during the electropolishing process, nozzle 106 acts as a cathode, and wafer 100 acts as an anode. When stream of electrolyte 108 is applied to metal layer 102, the difference in potential between electrolyte 108 and metal layer 102 results in the electropolishing of metal layer 102 from wafer 100. Although power supply 110 is depicted as being directly connected to wafer 100, it should be recognized that any number intervening connection can exist between power supply 110 and wafer 100. For example, power supply 110 can be connected to chuck 112, which is then connected to wafer 100, and, more particular to metal layer 102. For an additional description of electropolishing, see U.S. patent application Ser. No. 09/497,894, entitled METHOD AND APPARATUS FOR ELECTROPOLISHING METAL INTERCONNECTIONS ON SEMICONDUCTOR DEVICES, filed on Feb. 4, 2000, which is incorporated herein by reference in its entirety.
In the exemplary embodiment depicted in
Although in the exemplary embodiment depicted in
With reference to
With reference to
As depicted in
In the present exemplary embodiment, after metal layer 102 has been polished to intermediate thickness 304, nozzle 106 (
As depicted in
As noted above, with reference again to
With reference to
In the present exemplary embodiment, as depicted in
As depicted in
For example, assume metal layer 102 is copper and underlying layer 306 is a barrier layer, which is typically Ta, TaN, Ti, TiN, W, WN. Because the resistivity of barrier layer 306 is typically ten to hundred times higher than that of copper, the polishing rate on a portion of metal layer 102 that is discontinuous with portions of the underlying barrier layer 306 exposed is much lower than if the portion was continuous without any of the underlying barrier layer 306 exposed.
As depicted in
With reference to
In the present exemplary embodiment, as depicted in
As depicted in
After underlying layer 306 has been initially exposed at a point, in a second stage, the stream of electrolyte is applied from the nozzle onto additional portions of metal layer 102 extending from the center portion toward the edge portion of wafer 100. As will be described in more detail below, in the present exemplary embodiment, during this second stage, the shape of the polishing profile can be adjusted.
For example, as depicted in
With reference again to
With reference again to
With reference again to
With reference to
V(x) is the lateral relative speed or velocity. C is a constant. x is a radial location from the center of wafer 100 in the x-direction in the coordinate system depicted in
However, as described above, with reference to
For example,
Thus, in one exemplary embodiment, the polishing profile is tuned to match the thickness profile of a wafer. In particular, with reference to
In addition to fluctuations in the thickness of metal layer 102 across wafer 100 at different radial locations, the thickness of metal layer 102 can vary at different circumferential locations (theta locations) at a particular radial location on wafer 100 due to pattern sensitivity. For example, the thickness of metal layer 102 at a particular point on wafer 100 located at a radial location and at a theta location can differ from another point on wafer 100 located at the same radial location but at a different theta location, in part, because the two points have different wire patterns underneath metal layer 102.
Thus, in one exemplary embodiment, a first set of averaged thicknesses at different radial locations on wafer 100 is calculated of thicknesses at two or more points at the same radial location but different theta locations on wafer 100. A second set of averaged thicknesses at different radial locations on wafer 100 are then calculated using two or more of the averaged thicknesses from the first set of averaged thicknesses. The second set of averaged thicknesses is then used as the thickness profile of metal layer 102 in varying the lateral relative speed between wafer 100 and nozzle 106.
For example, with reference to
In one exemplary embodiment, a lateral relative speed compensation factor at a radial location on the wafer is determined based on the second set of averaged thicknesses at the different radial location on the wafer. The lateral relative speed between the wafer and the nozzle at a radial location can be determined by the lateral relative speed compensation factor at the radial location. Lateral relative speed compensation factors across the wafer can then be compiled as a lateral relative speed compensation factor curve for the wafer.
For example, a lateral relative speed compensation factor can be calculated using the following formula:
X(x)=(Ts(x)/Ta(x))α (2)
X(x) is the lateral relative speed compensation factor. x is the radial location from the center portion on the wafer. Ts(x) is a thickness of the metal layer at a radial location resulting from electropolishing the metal layer without varying the lateral relative speed, such as the thicknesses depicted in
With reference to
With reference again to
The viscosity of electrolyte is determined by two primary parameters: (1) temperature of the electrolyte; and (2) the composition of the electrolyte.
In a typical electropolishing electrolyte, which is acid base, salt base, or alkali base, water is easily removed from, or added into, the electrolyte by evaporation or absorption. An increase in the water content in the electrolyte will generally result in a reduction of the viscosity of the electrolyte.
Thus, in one exemplary embodiment, to maintain a constant polishing rate, a constant viscosity of the electrolyte in the stream of electrolyte is maintained as the stream of electrolyte is applied onto the metal layer between the center portion and the edge of the wafer. In the present exemplary embodiment, the viscosity of the electrolyte is maintained constant by measuring the water content in the electrolyte and controlling a water-to-electrolyte balance in the electrolyte based on the measured water content in the electrolyte.
The water content in the electrolyte can be measured using a temperature compensated viscosity (Tcv) meter. As depicted in
With reference to
In the present exemplary embodiment, electrolyte outlets 2514, 2516, 2518 supply electrolyte to one or more nozzles 106 (
In the present exemplary embodiment, the temperature within electrolyte reservoir 2502 is set at a certain level (a temperature set point) so that the water evaporation rate is slightly higher than the water absorption rate. The water content in the electrolyte can be maintained at a constant by dosing water into electrolyte reservoir 2502 through water dosing inlet 2522 using water dosing control valve 2524.
Note that the absorption rate and evaporation rate can depend on ambient moisture and temperature surrounding the electrolyte and/or electrolyte reservoir 2502. For example, for phosphoric-based electrolyte, the water evaporation rate is higher than water absorption rate if the temperature of electrolyte reservoir 2502 is set at 35° C. with ambient temperature of 20° C. and ambient moisture at 70%.
In the present exemplary embodiment, processor 2508 sends the temperature set point to temperature control unit 2510. Temperature control unit 2510 then adjusts its heating/coolant temperature based on the reading from temperature sensor 2506. The control mechanism used can be a typical proportion, integration, and deviation (PID) control process.
Viscosity meter 2504 sends a Tcv reading back to processor 2508. Processor 2508 sends signals to turn on water dose valve 2524 if the Tcv is lower than the temperature set point. The dose amount can be set based on pre-calibration data, such as the relationship between water content and Tcv depicted in
By using the closed water dose control mechanism described above, the water content can be measured and controlled at a certain value with minimum deviations. By controlling the water content, the physical viscosity of the electrolyte, and in turn the polishing rate, can be controlled.
In another exemplary embodiment, rather than measuring Tcv, viscosity meter 2504 can measure the physical viscosity of the electrolyte in electrolyte reservoir 2502. Viscosity meter 2504 sends the physical viscosity measurement to processor 2508. If the physical viscosity of the electrolyte is higher than a set point, processor 2508 sends a lower temperature set point to temperature control unit 2510. If the physical viscosity of the electrolyte is lower than a set point, processor 2508 sends a higher temperature set point to temperature control unit 2510. The appropriate temperature set point can be determined based on pre-calibrated data, such as the relationship between temperature and physical viscosity depicted in
Note that a constant physical viscosity can be maintained during a brief duration by adjusting temperature. A constant physical viscosity can be maintained for a longer duration by maintaining a constant water content in the electrolyte.
In one exemplary embodiment, a constant flow rate of the electrolyte is maintained in the stream of electrolyte as the stream of electrolyte is applied onto the metal layer between the center portion and the edge portion of the wafer. As described above, with reference to
With reference to
In the present exemplary embodiment, a pump 2604, which is operated by compressed air, pumps electrolyte from electrolyte reservoir 2502. As depicted in
With reference to
With reference to
With continued reference to
With reference to
When electrolyte is to be supplied to polishing chamber 2602, second pneumatic ON/OFF valve 2620 is closed, while both first pneumatic ON/OFF valve 2612 and control valve 2614 are opened. When electrolyte is to be supplied back to electrolyte reservoir 2502 while bypassing polishing chamber 2602, control valve 2614 is closed, while both first pneumatic ON/OFF valve 2612 and second pneumatic valve 2620 are opened. Note that when control valve 2614 is closed and first and second pneumatic ON/OFF valves 2612, 2620 are opened, electrolyte in the supply line between control valve 2614 and first pneumatic ON/OFF valve 2612 can drain back to electrolyte reservoir 2502.
In one exemplary embodiment, a look-up table is used to determine the appropriate pressure of pilot air to control valve 2614 to cause it to pass the appropriate amount of electrolyte to achieve the desired flow rate. The following describes a process by which processor 2618 generates the look-up table:
-
- 1. Processor 2618 sends command to pneumatic pressure regulator or IP converter 2616 to generate one Nth of full pressure P0. N is an integer, which preferably is in a range between 5 and 100, and more preferably is 30.
- 2. Processor 2618 records the flow rate measured by flow meter 2610 through A/D converter 3002.
- 3. Processor 2618 sends command to pneumatic pressure regulator or IP converter 2616 to generate two Nth of full pressure.
- 4. Processor 2618 records the flow rate measured by flow meter 2610 through A/D converter 3002.
- 5. Repeats steps 3 and 4 for additional points 3, 4, . . . , N-1, N separately.
The resulting look-up table is depicted below:
Once the look-up table has been generated, for a desired flow rate (f0), processor 2618 can search the look-up table for an entry with a matching flow rate to determine the appropriate pressure set point to provide to pneumatic pressure regulator or IP converter 2616.
If the desired flow rate (f0) is not in the look-up table, processor 2618 interpolates between at least two points in the look-up table. In particular, processor 2618 finds a range f(n−1) and f(n) such that f(n−1)<f0<f(n). Processor 2618 then calculates an initial pressure set point P1 as follows:
P1−P0*(n−1)/N+(f0−f(n−1))*((P0*n/N)−P0*(n−1)/N))/(f(n)−f(n−1)) (3)
The initial pressure set point P1 is sent to pneumatic pressure regulator or IP converter 2616, which then supplies pressure P1 to control valve 2614 to produce an initial flow rate (f1).
If f1 is sufficiently different from f0, such as beyond an established margin of error, the following formula can be used to adjust the flow rate again:
P2=P1+(f0+f1)*((P0*n/N)−P0*(n−1)/N))/(f(n)−f(n−1)) (4)
Additional flow measurements are then repeated obtained from flow meter 2610 to adjust the pressure being supplied to control valve 2614 to maintain a flow rate closest to the desired set point.
Note that the look-up table can be regenerated or updated periodically depending on the stability of control valve 2614, A/D and D/A converter 3002, and pneumatic pressure regulator or IP converter 2616. Note also that the process described above is useful when upstream or downstream pressure varies during the polishing operation.
Thus, in one exemplary embodiment, the temperature of the electrolyte is measured. The polishing current applied to the stream of electrolyte is then adjusted based on the temperature of the electrolyte. For example, when the temperature of the electrolyte increases, the polishing current can be reduced to compensate. In particular, the polishing current can be set as follows:
I0 is the set point of the polishing charge. T0 is the temperature set point. dT is the temperature deviation from the temperature set point T0. ρ(T, I) is the polishing efficiency function.
With reference to
Thus, in one exemplary embodiment, gas bubbles are removed from the electrolyte in electrolyte reservoir 2502 before pumping the electrolyte back to the nozzle from electrolyte reservoir 2502. In particular, with reference to
As depicted in
In particular, the electrolyte flows back into electrolyte reservoir 2502 through return inlet 2520. The electrolyte flows from electrolyte return inlet 2520 through a first channel in a first direction. The electrolyte flows from the first channel into a second channel in a second direction, which is in the opposite direction from the first direction. The electrolyte flows from the second channel into a third channel in a third direction, which is the opposite direction from the second direction and in the same direction as the first direction. The electrolyte then flows from the third channel into outlets 2514, 2516, and 2518, which are located near the bottom of electrolyte reservoir 2502 to further reduce the likelihood of gas bubbles being pumped back to the nozzle. Heating/cooling elements 3106 can be disposed within the channels.
By prolonging the return of the electrolyte before pumping the electrolyte back to the nozzles, any gas bubbles in the electrolyte has enough time to rise to the surface of the electrolyte. See also, U.S. Provisional Patent Application Ser. No. 60/462,642, filed on Apr. 14, 2003, which is incorporated herein by reference in its entirety.
Although various exemplary embodiments have been described, it will be appreciated that various modifications and alterations may be made by those skilled in the art. For example, the various concepts described above can be used with an electropolishing device that uses an applicator that directly contacts the metal layer rather than a nozzle that directs a stream of electrolyte without directly contacting the metal layer.
Claims
1. A method of electropolishing a metal layer formed on a wafer, the wafer having a center portion and an edge portion, the method comprising:
- aligning a nozzle and the wafer to position the nozzle adjacent to the center portion of the wafer;
- rotating the wafer; and
- as the wafer is rotated, applying a stream of electrolyte from the nozzle onto a portion of the metal layer adjacent to the center portion of the wafer to begin to electropolish the portion of the metal layer with a triangular polishing profile to initially expose an underlying layer underneath the metal layer at a point.
2. The method of claim 1, wherein the metal layer includes copper, and wherein the underlying layer is a barrier layer.
3. The method of claim 1, further comprising:
- after the underlying layer has been initially exposed at a point, applying the stream of electrolyte from the nozzle onto additional portions of the metal layer extending from the center portion toward the edge portion of the wafer; and
- adjusting the triangular polishing profile to have a flatter apex when the stream of electrolyte is applied to the additional portions of the metal layer.
4. The method of claim 3, wherein adjusting the triangular polishing profile comprises:
- applying a first polishing current to the stream of electrolyte when the stream of electrolyte is applied to the portion of the metal layer adjacent to the center portion of the wafer; and
- applying a second polishing current, which is higher than the first polishing current, when the stream of electrolyte is applied to the additional portions of the metal layer.
5. The method of claim 3, wherein adjusting the triangular polishing profile comprises:
- applying the stream of electrolyte using a first nozzle when the stream of electrolyte is applied to the portion of the metal layer adjacent to the center portion of the wafer; and
- applying the stream of electrolyte using a second nozzle, which is larger than the first nozzle, when the stream of electrolyte is applied to the additional portions of the metal layer.
6. The method of claim 1, wherein the wafer and the nozzle are not moved in a lateral direction when the stream of electrolyte is applied to the portion of the metal layer adjacent to the center portion of the wafer until the underlying layer is initially exposed at a point.
7. The method of claim 6, wherein, when the underlying layer is initially exposed at a point, the wafer or nozzle is moved in a lateral direction to apply the stream of electrolyte to additional portions of the metal layer extending from the center portion toward the edge portion of the wafer.
8. The method of claim 1, wherein aligning a nozzle adjacent to the center portion of the wafer comprises:
- moving the wafer to align the center portion of the wafer adjacent to the nozzle.
9. The method of claim 1, wherein aligning a nozzle adjacent to the center portion of the wafer comprises:
- moving the nozzle to align the center portion of the wafer adjacent to the center portion of the wafer.
10. The method of claim 1, wherein aligning a nozzle adjacent to the center portion of the wafer comprises:
- moving the nozzle and the wafer relative to one another to align the nozzle adjacent to the center portion of the wafer.
11. A system to electropolish a metal layer formed on a wafer, the wafer having a center portion and an edge portion, the system comprising:
- a wafer chuck to rotate the wafer; and
- a nozzle,
- wherein the nozzle and the wafer are aligned to position the nozzle adjacent to the center portion of the wafer, and
- wherein, as the wafer is rotated, a stream of electrolyte is applied from the nozzle onto a portion of the metal layer adjacent to the center portion of the wafer to begin to electropolish the portion of the metal layer with a triangular polishing profile to initially expose an underlying layer underneath the metal layer at a point.
12. The system of claim 11, wherein the metal layer includes copper, and wherein the underlying layer is a barrier layer.
13. The system of claim 11,
- wherein, after the underlying layer has been initially exposed at a point, the stream of electrolyte is applied from the nozzle onto additional portions of the metal layer extending from the center portion toward the edge portion of the wafer, and
- wherein the triangular polishing profile is adjusted to have a flatter apex when the stream of electrolyte is applied to the additional portions of the metal layer.
14. The system of claim 13, further comprising a power supply configured to:
- apply a first polishing current to the stream of electrolyte when the stream of electrolyte is applied to the portion of the metal layer adjacent to the center portion of the wafer; and
- apply a second polishing current, which is higher than the first polishing current, when the stream of electrolyte is applied to the additional portions of the metal layer.
15. The system of claim 13, wherein the nozzle comprises:
- a first nozzle configured to apply the stream of electrolyte when the stream of electrolyte is applied to the portion of the metal layer adjacent to the center portion of the wafer; and
- a second nozzle configured to apply the stream of electrolyte when the stream of electrolyte is applied to the additional portions of the metal layer, wherein the second nozzle is bigger than the first nozzle.
16. The system of claim 11, wherein the wafer and nozzle are not moved in a lateral direction when the stream of electrolyte is applied to the portion of the metal layer adjacent to the center portion of the wafer until the underlying layer is initially exposed at a point.
17. The system of claim 16, wherein, when the underlying layer is initially exposed at a point, the wafer or nozzle is moved in a lateral direction to apply the stream of electrolyte to additional portions of the metal layer extending from the center portion toward the edge portion of the wafer.
18. The system of claim 11, further comprising a guide rod configured to move the wafer to align the center portion of the wafer adjacent to the nozzle.
19. The system of claim 11, further comprising a guide rod configured to move the nozzle to align the center portion of the wafer adjacent to the center portion of the wafer.
20. The system of claim 11, further comprising:
- a first guide rod configured to move the nozzle; and
- a second guide rod configured to move the wafer.
21-87. (canceled)
Type: Application
Filed: Feb 23, 2005
Publication Date: Jun 7, 2007
Applicant: ACM RESEARCH INC. (FREMONT CALIFORNIA, CA)
Inventors: Hui Wang (Fremont, CA), Afnan Muhammed (Fremont, CA), Jian Wang (Fremont, CA), Felix Gutman (San Jose, CA), Frederick Ho (San Jose, CA), Himanshu Chocshi (Fremont, CA)
Application Number: 10/590,460
International Classification: B23H 3/00 (20060101);